It is frequently necessary to control from a central location a plurality of loads which are located at separate remote locations. An example of such a system would be a heating system of a building wherein the separate rooms and areas have their own individual thermostats and their own individual heating needs. In such an arrangement it is necessary to operate some of the loads during predetermined periods, other loads during other periods, and to provide a priority system whereby certain loads can be operated at any time. In such a heating system, for instance, certain areas are heated only during the day, and other areas are heated to a lower temperature at night than during the day.
Traditionally such control has been exercised by providing a central master controller which is connected to each slave unit operating a respective load via a respective conductor or pair of conductors. Thus in such a system it is necessary to provide a very large multiconductor cable that passes through the slave units and connects them all back to the master controller. In addition it is almost impossible to expand such a unit, as the addition of each new slave unit requires the wiring back to the master controller of a new conductor or pair of conductors.
This difficulty has been overcome in some systems by providing complicated coding arrangements at the master controller and complicated decoders at the slave units. Thus it is possible to pass a single conductor or pair of conductors from the master controller to all of the slave units. A pulse-coded signal is formed at the master controller and is decoded at each of the slave units so that only those units programmed to operate with a certain code will be operated at certain times. Such a system is relatively expensive and failure-prone, as pulses induced in the wiring by external forces can frequently cause operation of the wrong slave units or nonoperation of the slave units that are supposed to operate. Another difficulty with such systems is that an extremely complicated second decoding arrangement is necessary if an override provision is required at one of the slave units.
Another disadvantage of the prior-art systems is that each slave unit requires its own power supply. This power is frequently taken directly off the load, but in any case it is necessary that the separate slave units be connected to and provided with their own power supplies. The result is a considerable increase in the cost of each slave unit, especially in systems where the load is a 3-phase 220 volt heater, motor, or the like.